In a machine which comprises a movable section, connected to a servomotor by means of a rotary-motion-to-linear-motion converter mechanism or the like, and a rotary position sensor attached to the servomotor, as is generally known, the position of the machine movable section is detected according to feedback pulses delivered from the position sensor accompanying the motor rotation, while the machine movable section is being moved by means of the servomotor with the aid of the converter mechanism or the like, whereby position control of the machine movable section is effected. Also known is an arrangement such that position control for a target moved position is finished when the actual moved position of the machine movable section, detected according to the feedback pulses, enters an in-position zone which contains the target position, and a warning is issued to the effect that a deviation between the target moved position and the actual moved position is too large when the positional deviation departs from an allowable range. In consideration of the allowable ranges for the in-position zone and the positional deviation, it is advisable to set a movement of the machine movable section corresponding to the number of input feedback pulses at a value divisible by the pulse number. Meanwhile, a movement of the machine movable section for each revolution of the servomotor varies depending on the equipment arrangement, such as the screw pitch of the rotary-motion-to-linear-motion converter mechanism, the gear ratio of a speed reduction mechanism, etc.
Conventionally, therefore, a position sensor, such as a pulse coder, is used which has a resolution high enough to fulfill the aforementioned requirements for the relationship between the movement of the machine movable section, such as a table, and the feedback pulse number. Used in a machine whose table movement for each revolution of the servomotor is 6 mm, for example, is a pulse coder of a type such that 3,000 pulses are generated with every servomotor revolution in each of two phases with a phase difference of 90.degree., and feedback pulses are delivered at the leading and trailing edges of each of the generated pulses. In this case, 12,000 feedback pulses are delivered from the pulse coder with every motor revolution, so that the resolution (table movement per one feedback pulse) of the pulse coder is 0.5.mu.. Used in a machine whose table movement for each motor revolution is 8 mm or 10 mm, according to another example, is a pulse coder of a type such that 8,000 or 10,000 feedback pulses are generated with every motor revolution, that is, the resolution is 1.mu. or 0.5.mu..
The conventional method described above uses a pulse coder whose resolution is suited for the specifications of the machine and servomotor. When controlling the operations of machines and servomotors of various types, therefore, various pulse coders of different resolutions must be provided for use. Naturally, moreover, the types of available pulse coders are limited, so that the machines and servomotors to which the aforementioned conventional method are limited. Furthermore, the management of the machines, servomotors, and pulse coders is complicated.
As another method which fulfills the aforementioned requirements for the relationship between the movement of the machine movable section and the feedback pulse number, use of a control system (FIG. 3) including an optional CMR (command multiplied ratio) 6, whose operation characteristic can be variably adjusted so as to change the significance of a position command depending on the resolution of the pulse coder, is generally known. If a pulse coder 4 which generates one feedback pulse for each table movement of 1.mu. is used, for example, the operation characteristic of the CMR 6 is set so that one movement command pulse is delivered from the CMR when a movement command (variation of position command) of 1.mu. is applied to the input of the CMR. If another pulse coder 4 which generates, e.g., 2.5 feedback pulses for each table movement of 1.mu. is used, moreover, the operation characteristic of the CMR 6 is set so that two or three movement command pulses are alternately generated, and therefore, 2.5 movement command pulses are delivered on the average when 1-.mu. movement commands are inputted in succession.
Even though the movement command is fixed, however, different numbers of movement command pulses are delivered from optional CMRs 6 with different operation characteristics. According to the aforementioned conventional method in which the operation characteristic of the CMR 6 is variably set, therefore, the in-position zone, used to discriminate the arrival at the target position, and the allowable range, used to detect an excessive positional deviation, must be modified every time the operation characteristic of the CMR 6 is changed. Naturally, moreover, the setting range for the operation characteristic of the CMR 6 is limited, so that available pulse coders for the execution of the aforementioned conventional method are limited.